1. Field of the Invention
The present invention generally relates to a Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) made on semiconductor on insulator (SOI) substrates. In particular, the invention relates to MOSFETs with a work function adjusted metal backgate.
2. Description of Related Art
Work function is a material property defined as the energy needed to move an electron from the Fermi level to vacuum. The work function of a material is of interest in MOSFET applications because tuning the work function of a top gate electrode defines the threshold voltage of the MOSFET and influences the transistor performance.
A top gate electrode comprises a gate dielectric and a gate conductor, usually a metal. To change the top gate work function at least the following approaches have been used: using different metals or metal alloys, adjusting metal thicknesses, manipulating metal composition, or manipulating the gate dielectric so that it contains certain amount of fixed or dipole charges.
In a planar fully-depleted semiconductor-on-insulator (FDSOI) device, such as extremely-thin semiconductor-on-insulator (ETSOI) MOSFETs, it is also possible to adjust the threshold voltage of a transistor by inserting a backgate underneath the device and which is separated from the channel by a thin buried oxide (BOX).
Usually the backgate is formed by implanting dopant species into the semiconductor substrate to form a doped region underneath the BOX. Depending on the polarity of the doped region and the thickness of the buried oxide, the threshold voltage of the transistor can be adjusted.
However, absent a voltage, the Fermi Level of a heavily doped semiconductor region can be only one of the two values that correspond to the conduction band and valence band of the substrate. Therefore without a voltage applied to a doped semiconductor backgate, the doped semiconductor backgate cannot adjust the threshold voltage by an arbitrary value smaller than what is determined by the difference between the conduction band and valence band of the substrate.
To apply a voltage to a semiconductor backgate, contacts must be made to the substrate and isolation regions must be formed between the backgates of different MOSFETs. Furthermore, in order to shift threshold voltages by an arbitrary value, voltages other than ground and the supply voltage are needed. That means circuits are needed to both generate such voltages and to route the voltage to the appropriate contacts throughout the chip. The need for additional contacts, isolation areas, and circuits increases the complexity of the fabrication process and increased chip area.